Cold cathode electron discharge device and circuits therefor



pt. 7, H948., c. w. 'HANsELL COLD CATHODE ELECTRON DISCHARGE DEVICE ANDCIRCUITS TBEREFGR- Filed Spt. 8, 1944 3 Sheets-Sheet l pi. 7, 48. C, w,HANSELL 2,448,527 GOLD GATHODE ELECTRO DISCHARGE DEvIcE AND CIRCUITSTHEREFQR Filed Sept. 8. 1945. 3 Sheets-Sheet 2 @fyi " l-a' r :n-i

ATTORNEY' c. w. HANsELL 2,527 GOLD CATHODE ELECTRO DISCHARGE DEVICE ANDCIRCUITS TBEREFOR 3 Sheets-Sheet 3 Filed Sept. 8, 1944 TOR/VE Y linderclosed at both ends.

Patented Sept. 7, 1948 1 COLD CATHODE ELECTRON DISCHARGE DEVICE ANDCIRCUITS THEREFOR ciarence w. Hansen, rortreierson, N. Y., asmgnor toRadio Corporation of America, a corporation of Delaware ApplicationSeptember 8, 1944, Serial No. 553,138

(Cl. Z50-36) 14 Claims. i

The present invention relates to electron disratus for efcientlyconverting low Vfrequency or direct current energy to audio or radiofrequency energy.

Another object is to provide a simple and rugged vacuum tube structureand circuit capable of handling power levels ranging up to hundreds ofkilowatts, with relatively high eiliciency.

A further object is to provide a high power evacuated electron dischargedevice which requires no great concentration of electron emission.

A still further object is to provide a magnetron type of generatorhaving a cold cathode and whose individual elements have relativelylarge surface areas as a result of which the eects of temperaturev areminimized.

A still further object is to provide a magnetron type of oscillationgenerator for use in communication, in which the tube employs a coldsecondary emissive cathode lying in the axis of a cavity resonatorconstituting an anode for the tube.

in one embodiment of the invention, the electron discharge device is avacuum tube which employs a relatively cold tubular cathode. Thiscathode is cooled in suitable manner and is so arranged as to producecopious secondary electrons when bombarded. Means are provided toproduce a magnetic field in a direction parallel to the axis of thecathode. A cavity resonator 'in which appears a periodically reversingelectromagnetic field surrounds the cathode. This cavity resonator is inthe form of a metallic cyl- A hollow cathode passes through these endsbut is insulated therefrom by suitable seals, such as glass. In effect,the cavity resonator serves also as an anode, and this anode is suppliedwith a power from a source of controllable direct or low frequencypotential and current. A source of relatively high frequency power isutilized to excite the tube to produce oscillations. Theanode-to-cathode potential is periodically increased above and reducedbelow what would be the cut-od potential value in the absence ofoscillations, but at a frequency lower than the excitation frequency. Asa result of this circuit arrangement, electron current flows betweenanode and cathode in the form of pulses of a frequency related to theperiodic rise and fall of the anode-to-cathode potential.

The generator circuit of the invention may 2 have wide industrial uses,suchas for example in the induction heating iield. If desired, theinvention can be used for communication purposes on frequencies rangingfrom a relativelylow value up to perhaps 100 megacycles or more,provided the excitation frequency is properly chosen in relation to theoutput frequency. If employed for communication purposes, the system ofthe invention can be modulated in a manner described hereinafter.

A more detailed description of the invention follows in conjunction withthe drawings, wherein: v

Figs. 1 and 2 illustrate two embodiments of cold cathode oscillators, inaccordance with the present invention; i

Fig. la illustrates a cross-sectional view of the magnetron of Fig. 1and shows the end walls and uid cooling arrangement in more detail;

Figs. 3 and 3a, are graphs given to explain the operation of the presentinvention; and

Fig. 4 shows an embodiment of the invention applicable to radiocommunication.

Referring to Fig. 1 in more detail, there is shown a, cross section of amagnetron construction including a hollow cylindrical cathode l,surrounded by a metallic cavity resonator 2. The cathode l lies in theaxis of the tube and extends from both ends of the cavity resonator.This cavity resonator is a metallic cylinder` closed at the ends (notshown) to provide an electrically closed chamber in which there isarranged to appear a periodically reversing electromagnetic eld. Thecathode is insulated from the cylinder by means of suitable insulatingseals, such as glass. For cooling the cathode water is preferablysupplied through the insulating seals so as to flow through the interiorof the cathode in order that the cathode be the coolest surface exposedto the vacuum. Such a water cooling arrangement is shown in Fig. 1a. andalso in my copending application Serial No. 534,066, led May 4, 1944,now U. S. Patent 2,420,744 granted May 20, 1947. The space between thecathode l and the inner walls of the cavity resonator 2 is evacuated inthe manner of any well known vacuum tube device. Surrounding the cavityresonator 2 there is provided a coil 3 energized from a unidirectionalsource t so as to provide a magnetic eld in the direction parallel tothe cathode. If desired, coil 3 can be replaced by a permanent magnethaving field poles placed at the ends of the cavity resonator adjacentthe seals, so as to produce a, magnetic field in a direction parallel tothe cathode. The cavity resonator, by virtue of its metallic surface,serves as the anode of the tube structure.

A source of controllable direct or low frequency potential and power 5has its negative terminal connected through ar parallel tuned circuit 6to the cathode while the positive terminal of the source 5 is connectedto the cavity resonator, as shown. A bypass condenser 1 is connectedacross the positive and negative terminals of source 5. A source 8 ofvery high frequency power is coupled through a suitable glass seal 8 tothe interior of the cavity resonator by means of a coupling loop I0.Source 8 may, for example, be of the order of 1000 megacycles and servesto excite the' generator. Tuned circuit 8 may be tuned to a low radiofrequency of the order of one megacycle, by way of example, or even to ahig-h audio frequency. The cavity resonator 2 is arranged to be resonantto a frequency of 1000 megacycles, which is the frequency of source t.This magnetron may, as a preferred example, be so dimensioned that it isresonant for 4a mode of cavity resonancein which a circumferentialelectric eld may be set up at the frequency of source 8 which is muchhigher than the frequency of the main oscillations in tuned circuit 6.Preferably this is done by making the cavity resonator 2 the frequencycontrolling element in the oscillator. Output energy oi relatively lowradio frequency, for example, one megacycle, is derived from the tunedcircuit 6 by means of coil Ii, which is coupled to the inductance oftuned circuit 6. By way of illustration only, the output coil ll isshown connected to a typical induction furnace arrangement comprising acrucible within a heat insulation chamber.

The strength of the high frequency electric eld is made to be adjustablein order to obtain best operation, but need not be very great. totalpeak electric eld potential around the whole circumference at the lineof maximum iield, of a,

few thousand volts should suce to cause a large portion of anycirculating electrons present between cathode and anode to gain enoughenergy from the electric field to strike the cathode with suflicientvelocity to produce substantial secondary emission from the cathode i'.The magnetic eld is preferably adjusted to give resonance of electronmotion at the frequency of the source 8. In this way there is someco-relation between the magnetic field and the frequency of the excitingsource 8 for easiest and most efcient operation. This co-relation,however, is not essential, provided the strength of the driving electrici'leld supplied by source 8 is suiiiciently great to give the requiredenergy to the electrons to bombard the cathode.

To understand the operation of the invention, it should be noted that solong as the anode-tocathode potentialprovided by source 5 is above thecut-o potential corresponding to the particular strength of magneticfield used, and the magnetron dimensions, so that all electrons emittedfrom the cathode are directly drawn to the anode on their rst trip fromthe cathode, then any electron 'released from the cathode or released atany point between the cathode and the anode will be pulled immediatelyto the anode and thereby removed from the free vacuum. The possible pathof such an electron is illustrated by the dash line i2, which shows whathappens when the anode-to-cathode potential is so high (above cut-off)that all electrons are directly drawn to the anode on their first trip.When this occurs, the anode-to-cathode potential is considered to beabove the cut-off potential. Because the cathode is unheated, andtherefore has extremely low electron emission, and the vacuum betweenthe cathode and anode is supposed to be good, it will be evident thatonly negligible currents can flow between the anode and the cathode whenthe anode-to-cathode potential is much above the cut-off potential.However, when the anode-to-cathode potential is low, below the cut-oil?potential, then electrons leaving the cathode will in whole or in partbe returned to the cathode by the magnetic eld and will lfollow somesuch paths as indicated by the curved dash lines I3. When the electronsleave the cathode and move out toward the anode and back again, someelectrons will be accelerated by the Very high' frequency eld producedby source 8 in such manner that they arrive back at the cathode withexcess velocity and potential and cause 'secondary emission from thecathode. The curved dash lines i4 indicate the paths of a plurality ofsecondary electrons which are released from the cathode in response tobombardment from an electron in'one of the paths i3 reaching thecathode. The secondary electrons in turn may traverse the evacuatedspace and some of them will arrive back at the cathode i with excesspotential. Thus an extremely small electron emission may be multipliedvery rapidly until it becomes a large space charge limited emission.During this process, those electrons which lose energy due to the veryhigh frequency field (mainly out-of-phase electrons) and which do not.return all the way to the cathode, will strike the anode instead andmay constitute a very large anode current. This anode current isaugmented by electrons forced out to the end plates of the closedcylindrical anode, by space charge repulsion causing motions more orless parallel to the magnetic eld. In this way there is produced acondition of anode input current ow only when the anode-to-cathodepotential is low.

If now, the anode-to-cathode direct current input potential supplied bysource 5 is adjusted so as to start with an optimum low potential fromthe anode power source, and this potential then raised, it will beevident from the foregoing that operation will start with large anodecurrent. As this direct current input potential rises to the critical orcut-oil value, emission from the cathode will suddenly stop (because nowall electrons are drawn directly to the anode 2) and theanode-to-cathode direct current will be interrupted, starting anoscillation in the output tuned circuit 6. The oscillation in tunedcircuit 6 causes a variation in potential which is then augmented by theinterruption of circulating electron current when the overall anodepotential is relatively high; that is, the superposition of thealternating potential of tuned circuit 6 on that of source 5 willmomentarily raise the total anode potential above cut-off. Theoscillation also causes a variation in potential which produces a ow ofcirculating electron current when the overall anode potential isrelatively low, caused by the reversal in polarity of the potentialacross tuned circuit 6 which pushes the `total effective anode potentialbelow the cut-oli value during a portion of each cycle of oscillation ofcircuit 6, and the oscillation strength will then increase to a limitingvalue. This process is more clearly understood from an inspection ofFigs. 3 and 3a which shows one particular condition of operation of thetube of Fig. 1.

amata? In Fig. 3, the three horizontal lines are respectively labeledZero voltage, Cut-olf .potential forthe magnetron, andthe "Normaloperating direct current potential of source 5." It will be seen thatthe direct current potential of source b is considerably above the cutofpotential for the magnetron, and hence at this value of potential allelectrons emitted from the cathode i will be drawn directly to theanode. The alternating potential of tuned circuit is shown in the formof a sine wave. It. is only when the alternating potential o'f the tunedcircuit o is in the negative direction that it will oppose the directcurrent potential of source 5 and produce an eective or total anodepotential which falls4 below the cut-01T potential for the magonetron.These points at which the total anode potential is below cut-off occurin the time interval t during which a circulating space charge ispermitted to build up in the magnetron and to produce pulses ofanode-to-cathode current. Fig. 3a graphically illustrates the pulses ofoutput cur# rent available in output coil il.

.after radio frequency oscillations have been l started by rst reducingthe potential of source 5 to less than the cut-off potential, then theanode potential from source E may be increased to its operating valuewhich would normally be much above what would be the cut-ofi value inthe absence of oscillations in tuned circuit t, and the strength ofoscillation will increase and high power input and power output can beobtained so long as the secondary emission ratio from the cathoderemains high enough for electron emission to reach a saturation, valuewhen the overall anode potential is a minimum.

lin practice I have found that apparently there is always some residualthough extremely small electron emission in magnetrons which is enoughto provide starting and growth of emission. This may be due to photoemission caused by light rays penetrating the glass parts of the tube.or due to the action of cosmic rays and also due perhaps to a continuousredistribution of vola tile materials from condensation andre-evaporation from the cathode surface accompanied by the release ofions and electrons. Experiments on the effect of temperaturedistribution have shown that even those vacuum tubes with the bestcommercially obtainable vacuum oontain traces of volatile materialswhich are continually evaporated and condensed at the surfaces. Bychanges in temperature distribution eiecting the distribution anddensity of these materials on the surfaces, it has been possible toproduce profound effects upon the operation, reliability, and break-downpotentials of vacuum tubes. For. this reason it lspreferred that the uphigh frequency electric nelds which, in conjunction with the anodedirect current potential and the magnetic eld, will make it possible forsecondary emission to grow may be used. By way of example, the very highfrequency input may be applied as a potential between the anode andcathode in the manner illustrated in Fig. 2. Those parts of Fig. 2 whichare the same as those in Figui have been given the same referencenumerals. 'It should be noted in Fig. 2 that the excitation sourceofvery high frequency power 8 is applied between the anode and cathodeby virtue of a very high .frequency input coupling circuit it, theprimary winding of which is coupled to the source a and the secondarywinding of which is coupled between the cathode 2 and thenegative'terminal of the source 5. The parallel tuned input circuit tcomprises an inductance coil and a variable condenser, but this variablecondenser is connected in series with .the by-pass condenser l, bothcondensers being tron transit time frequency may be made much greaterthan the excitation frequency, in which case growth of secondaryemission may take place due to the falling electric eld phenomenon incathode surface be so cooled as to become the coldest `surface exposedto the vacuum, thus causing high secondary emission to be main tained.

There also is evidence that cathode surfaces are made to show a largesecondary emission ratio as a result of th unnatural moleculararrangements formed by, ion bombardment. A hot cathode loses thisactivation almost as fast as it occurs but a cold cathode can hold itfor a relatively long time. Therefore, a small amount of ionicbombardment of the cold cathode may be responsible for part of itssecondary emission properties.

It is to be understood that the circuit of Fig. 1 is not dependent onthe use of a circumferential very high frequency eld. Any means ofsetting magnetrons which I have previously described in my copendingapplication Serial No. l477,062 filed hebruary 25, 1943, now U. S.Patent 2,427,781, granted September 23, 1947. The useof the fallingelectric field phenomenon requires higher excitation potentials to beused but has the advantage of. avoiding critical adjustments.

1n the practice of the present invention, it should be noted that thecathode emission is prodiced only when it is needed to supply anodeVcurrent. This is an advantage because it permits the obtainment ofrelatively high emission, thus producing emcient and low cathode powerdissiplation. By way of comparison only, it should be noted that inconventional tubes the cathode eii'iits continuously and requiresrelatively high heating energy which is avoided to a large degree in thepractice of the present invention.

'A further advantage of the present invention lies in the fact that theconstruction of the oscillator is simple, inasmuch as there is nonecessity for the use of control electrodes. Since the oscillator of theinvention is not dependent on thermionic emission from the cathode, itis not necessary to employ a, high emission density to obtain goodemission efficiency. For this reason it is possible to use in thepresent invention cathodes so large that to heat them for producingthermlonic emission according to conventional piractice would beentirely impractical. This feature of the invention allows the use ofrelatively small space charge densities and the attainment of highefllciencies. As mentioned above, it is preferred that the cathode bewater cooled in order to hold its operating surface at a'lowertemperature than any of the other surfaces exposed to the vacuum and Vtoaid in holding a large secondary emission ratio. Cooling the coldcathode 2 to such a low temperature tends to cause the volatilematerials in the vacuum tube to accumulate continually on the cathodesurface and thus help to maintain a high secondary emission ratio. Forexample, if caesium, rubidium, potassium, sodium, lithium, strontium,barium or thorium are present in the vacuum tube these materials willaccumulate on the cathode, and can, under suitable conditions, causevery large increases in secondary emission. At the present time thesematerials are not used in high voltage high power tubes of conventionaldesign because it has not been possible to keep a layer of them on a hotcathode under the required conditions. In accordance with the pres-v entinvention, however, by using a cold cathode whose operating surface ismaintained at a lower temperature than any of the other surfaces exposedto the vacuum in the tube, these materials can be kept mostly on thecathode. By maintaining the cathode colder than any other surface of thetube exposed to the vacuum, it is believed possible to use even highlyvolatile caesium to activate the cathode, and as the caesium isevacuated from the cathode surface partly by bom-A bardment, thismaterial may be continually replaced by condensation on the coldsurface. In this way, I am able to .provide greatly improved secondaryemission cathodes due to the control of the temperature distribution. Afurther advantage of caesium is that it tends to evaporate in the formof ions which will bombard the cathode surface to some extent, therebyhelping to maintain a surface full of electrical stresses of the kindneeded to produce a large secondary emission ratio.

If desired,.a small priming current for the cold cathode can be suppliedfrom a hot cathode and the amount of this priming current controlled bysuitable auxiliary electrodes. In this way I can obtain space chargelimited and controlled priming current rather than emission limitedpriming current.

A still further advantage of the present invention lies in the fact thatit provides a highly eiTlcient means for converting low frequency ordirect energy into audio or radio frequency energy.

In constructing the oscillator of the invention, the following conditionshould preferably be fullled: In addition to having a cold cathode withthe highest possible ratio of secondary emission, there should -be asuciently high ratio between the frequency of the cathode excitingcurrent and the oscillating frequency. 'I'his is because it takesincreased by a ratio of about 2. It a current of,

let us say, 100 amperesv or 100 coulombs per seccurrent, or

a little time for the growth of secondary emission and this time shouldbe a small fraction of the time of one cycle of output oscillations, asa result of which the time it takes for secondary emission to build upto an operating value should be small compared to the time period of acycle of tuned circuit 6 or 6', and preferably short compared with thetime periods in successive cycles during which it is desired thatanode-to-cathode current flow.

In orderto better understand VVthe invention, the following theoreticalreasoning will be given. It is to be understood.' however, that thistheoretical reasoning is merely given for the purpose of exposition andin order that the invention may be better appreciated. While thistheoretical exposition is believed to be correct, it is not of necessitycomplete, nor does the operation of the invention depend upon itsaccuracy or otherwise. Let us assume that the emission from the coldcathode may be made to double for each cycle of the very high frequencyexcitation potential. I f lwe consider n cycles after the growth ofemission begins, the emission current will then have ampere, where F isthe excitation frequency and 6.28X (10) 18 is the number of electronsrequired to make a coulomb, then:

log 2 For various Values 'of excitation frequency l,A the number ofcycles required to reach a current of amperes and the corresponding timein microseconds, assuming a single electron starts the growth on the rsteffective cycle, will be as given in the following table for the aboveassumption:

Number of i Excitation ireqluency in cyclxelsli) Mitmnds mcgacyc es reacamper 100 amperes As a refinement in the construction of the embodimentof Figs. 1 and 2, it may be found helpful to so arrange and adjust thelengths of the connections from the very high frequency source 8 and thecouplings that the source l delivers a maximum of excitation power aftersecondary emission current is built up and some lesser value whenemission is not active. This will hold down potentials when emission isnot required but hold them up -when emission is required, and take aminimum of average power from the source 8. As an alternative tosupplying excitation from a separate source such as 8, cr in addition toit, it may be possible to produce magnetron oscillations in the tube asan aid to producing secondary emission.

The invention, although described above particularlywith reference t0 apower converter is not limited to such use, since it can be used forcommunication purposes on relatively low frequencies. If used forcommunication, modulation may be accomplished by keying the potential ofsource 5 for telegraphy, or varying the resonant frequency of tunedcircuit 6 Afor producing frequency modulation. If desired,- thepotential of source 8 can be modulated to produce amplitude modulationof the output power. I may also `produce amplitude modulation byvariably absorbing the output power and by varying the coupling to theload, preferably with a saturable iron core modulator.

The term cold cathode used in this specification and the claims is usedto define a. cathode which is not heated directly from an externalsource, as distinguished from the conventional thermionic cathode whichis heated by electrical power from an external source.

Fig. 4 shows one embodiment of the invention which can be used for radiocommunication employing amplitude modulation. The potential of source ismodulated by source 2l through an audio transformer 22. Output fromtuned circuit 6 is fed to an antenna 2S. The parts of Fig. 4

which correspond with similar parts in Fig. 1 have been given the samereference numerals. The operation of the system of Fig. 4, will be0bvious from what has been stated above.

What is claimed is:

1. The method of operating a magnetron oscillator having a secondaryemissive cold cathode and a surrounding anode structure, which comprisesexciting said oscillator with very high frequency power of a frequencyhigher than the desired output frequency, and supplying said anode withan oscillating potential which oscillates between a value which ispositive relative to said cathode and much higher than what would be thenormal cut-od potential in the magnetron in the absence of outputoscillations to thereby inhibit the growth of secondary electrons fromsaid cathode, and a value below the normal cutoi potential but which isstill positive relative 'to said cathode at which last value there is agrowth of secondary electrons.

2. An electron dischargeldevice and circuit therefor comprising asecondary emissive cold cathode extending along the longitudinal axis ofsaid device, said cathode emitting electrons at a ratio greater thanunity when impacted by primary electrons, an anode surrounding saidcathode, there being an evacuated space between said cathode and anode,means providing a magnetic field whose ux lines are substantiallyparallel to said cathode, and means supplying said anode with apolarizing potential which is positive relative to said cathode andwhich periodically varies from a value above cut-ofl, at which secondaryemission from the cathode is inhibited to a value below cut-on` at whichthere is a growth of secondary emission, the time interval during whichthe potential on said anode is above the cut-off value being longer thanthe time interval during which it is below the cut-0H value.

3. An electron discharge device oscillator comprising a cold cathodecapable of emitting secondary electrons when bombarded by otherelectrons, said cathode extending along an appreciable portion of thelength of said device, a resonant anode structure surrounding saidcathode, means providing a magnetic field whose flux lines aresubstantially parallel to said cathode, a source of anode potentialcoupled to said anode through a tuned output circuit, means :forsupplying a relatively high frequency electric field to said oscillatorat a frequency greater than the resonant frequency of said tuned outputcircuit, said source supplying to said anode a potential which ispositive relative to said cathode and which is higher than what would bethe cutoff value in the absence of output oscillations, said tunedoutput circuit superimposing. alternating potential on the potential ofsaid source to thereby cause the total eiective anode potential toperiodically fall below the cut-oil value and enable the growth of totalelectron emission accompanied by pulses of electron current through thedevice.

4. An electron discharge device oscillator comprising a cold`cathodecapable of emitting secondary electrons upon bombardment by otherelectrons, said cathode extending along an appreciable portion of thelength 0f said device,

.a resonant anode structure surrounding saidl cathode, a source ofcontrollable direct current potential, and connections from said sourceto said anode and cathode for supplying said anode with a positivepotential relative to said cathode, a tuned output circuit connectedbetween said anode and said source, means for supplying a magnetic eldwhose flux lines extend substantially parallel to said cathode, a sourceof relatively hlgh frequency power of a frequency greater than thefrequency of said tuned output circuit, a coupling between said lastsource and said resonant anode structure arranged to produce acircumferential high frequency electric rleld in said oscillator, saidsource of direct current potential being adjusted to supply to saidanode a positive potential which is higher in value than what would bethe value of the cut-oli potential in the absence of oscillations, saidoutput tuned circuit superimposing an alternating potential on saiddirect .current source which causes the total effective anode potentialto oscillate at the frequency of said tuned circuit from a value abovecut-oi to a value below cut-off but still positive relative to saidcathode.

5.`The method of operating an electron discharge device having a coldsecondary emissive cathode, a surrounding resonant anode structure and amagnetic eld whose iux lines extend substantially parallel to saidcathode, which comprises exciting said oscillator with relatively highfrequency power of a frequency higher than the desired output frequency,supplying said anode with a low value of polarizing potential which ispositive relative to said cathode to thereby produce a large anodecurrent, raising said value of polarizing potential to the cut-oir valuefor said device at which the electrons strike the anode on their firstoutward trip, and then causing said anode potential to oscillate at theoutput frequency between values above and below said cut-off value.

6. An electron discharge device oscillator comprising a cold cathodecapable of emitting secondary electrons upon bombardment by otherelectrons, said cathode extending along an appreciable portion of thelength of said device, a resonant anode structure surrounding saidcathode, a source of controllable direct current potential, andconnectionsl from said source tol said anode and cathode for supplyingsaid anode with a positive potential relative to said cathode, a tunedoutput circuit connected between said anode and said source, means forsupplying a magnetic eld whose flux lines extend substantially parallelto said cathode, a. source of relatively high frequency power of afrequency greater than the frequency of said tuned output circuitcoupled to one of sai-d connections from said source of controllabledirect current potential for setting up a relatively high frequencyelectric eld in said oscillator, said source of direct current potentialbeing adjusted to supply to said anode a positive potential which ishigher in valuethan what would be the value of the cutoil` potential inthe absence of oscillations, said output tuned circuit superimposing analternating potential on said direct current source which causes thetotal effective anode potential to oscillate at the frequency of saidtuned circuit from a value above cut-ofi to a value below cut-off.

'7. A vacuum tube oscillator comprising a tubular cathode capable ofemitting secondary electrons when bombarded by other electrons,ametallic cavity resonator surrounding said cathode and constituting ananode, said cathode passing through opposite walls of said resonator andbeing insulated therefrom, means providing a magnetic eld with fluxlines parallel to said cathode, a cooling systemv for passing coolingfluid through said tubular cathode, a source of anode polarizingpotential coupled to said anode through a tuned output circuit, meansfor supplying a relatively high frequency electric eld to saidoscillator at a frequency greater than the resonant frequency of saidtuned output circuit, said source supplying to said anode a positivepotential which is higher than what would be the cut-off value in theabsence of output oscillations, said tuned output circuit supexcimposingalternating potential on the potential of said source to thereby causethe total effective anode potential to periodically fall bel-ow thecut-off value and enable the growth of total electron emission.

8. The method of operating a, magnetron oscillator having a secondaryemissive cold cathode and a surrounding anode structure, which comprisesexciting said oscillator with very high frequency power of a frequencyhigher than the desired output frequency, supplying said anode with anoscillating potential which osclllates between a value which is muchhigher than what would be the normal cut-oil' potential in the magnetronin the absence of output oscillations to thereby inhibit the growth ofsecondary electrons from said cathode, and a value below the normalcut-off potentiall relative to said cathode at which last value there isa growth of secondarly electrons, and supplying to said anode a maximumvalue of electron current during those intervals when sai-d oscillatingpotential is below the cut-oil' potential and a lesser value of electroncurrent during those intervals whenV said oscillating potential is abovethe cut-oil potential. 9. The method of operating a magnetron oscillatorhaving a secondary emissive cold cathode and a surrounding anodestructure, which comprises exciting said oscillator with very highfrequency power of a frequency higher than the desired output frequencyto provide a high ratio of exciting frequency to output frequency, andsupplying said anode with an oscillating potential which oscillatesbetween a value which is positive relative to said cathode and muchhigher than what woul-d be the normal cut-olf potential in the magnetronin the absence of output oscillations to thereby inhibit the growth ofsecondary electrons from said cathode, and a value below the normalcut-off potential but which is still positive relative to said cathodeat which last value there is a growth of secondary electrons, wherebythe time for the growth of secondary electrons to an operating value issmall compared to the time period of a cycle of said output frequency.

10. The method of operating an electron discharge device oscillatorhaving a secondary emissive cathode, a surrounding resonant anodestructure, and a substantially constant magnetic eld parallel to saidcathode, which includes applying a relatively high frequency potentialbetween anode and cathode as well as a very low frequency including zerofrequency potential in excess of what 'would be the cut-off value in theabsence of oscillations, and superimposing on said potential analternating potential which momentarily raises the total effective anodepotential considerably above the cut-off value and then reduces thetotal effective anode potential below the cut-off Value for a timeinterval which is small compared to the time period of a cycle o1 saidalternating potential.

1l. A transmitter for communication purposes comprising an electrondischarge device oscillator having a cold cathode capable of emittingsecondary electrons when bombarded by other electrons, said cathodeextending along an appreciable porti-on of the length of said device, aresonant anode structure surrounding said cathode, means providing amagnetic eld Whose flux lines are substantially parallel to saidcathode, a source of anode polarizing potential coupled to said anodethrough a. tuned output circuit, means for supplying a relatively highfrequency electric i field to said oscillator at a frequency greaterthan the resonant frequency of said tuned output circuit, said sourcesupplying to said anode a positive potential which is higher than whatwould be the cut-off value in the absence of output oscillations, saidtuned output circuit superimposing alternating potential on thepotential of said source to thereby cause the total effective anodepotential to periodically fall below the cut-off value and enable thegrowth of electron emission, and means for modulating the energyproduced in said tuned output circuit in accordance with theintelligence to be transmitted.

12. A transmitter in accordance with claim l1, characterized in thisthat said modulating means includes a circuit for modulating the directcurrent power input to the oscillator, to thereby control the time ofgrowth of total electron emission to an operating value.

13. A converter-of low frequency or direct cur-- rent power to higherfrequency energy suitable for use for induction heating purposes,comprising an evacuated electron discharge device having a cold cathodecapable of emitting secondary electrons when bombarded by otherelectrons, said cathode extending along an appreciable-portion of thelength of said device, a resonant anode structure surrounding saidcathode, means providing a magnetic field whose flux lines aresubstantially parallel to said cathode, a source of said power to beconverted connected between said anode and cathode for supplying saidanode with a polarizing potential, a tuned output circuit connectedbetween said anode and one terminal of said source, means for supplyingav relatively high frequency electric eld to said oscillator at afrequency greater than the resonant frequency of said tuned outputcircuit, said source supplying to said anode a positive potential whichis higher than what would be the cut-off value in the absence of outputoscillations, said tuned output circuit superimposing alternatingpotential on the potential of said source to thereby cause the totaleii'ective anode potential to peri odically fall below the cut-off valueand enable the growth of total electron emission.

14. An electron discharge device and circuit therefor comprising asecondary emissive cold cathode extending along the longitudinal axis ofsaid device, said cathode emitting electrons at a ratio greater thanunity when impacted by l primary electrons, an anode surrounding said lzcathode, there being an evacuated space 'between f said cathode andanode, means providing a magnetic iield whose flux lines aresubstantially parl allel to said cathode, and means supplyingl saidanode with a polarizing potential which is positive relative to saidcathode and which periodically varies from a value 'above cut-off, atwhich secondary emission from the cathode 1s inhibited Number Name Dateto a, value below cut-off at vwhich there is a 2,103,362. Hansen Dee,28, 193'? growth of secondary emission. 2,143,262 Farnsworth 'Jan. 10,1939 CLARENCE W.' HANSELL. 2,150,573 ZWorykin Mar. 14, 1939 5 2,159,521Farnsworth May 23, 1939 REFERENCES CITED 2,166,210 Fritz July 1s, 1939The following references are of record in the 2,196,392 Hansen APT- 91940 fue of this patent; 2,211,091 Braden Aug. 13, 1940 2,211,404 BradenAug. 13, 1940 UNITED STATES PATENTS 10 2,244,318 skenett June 3,1941Number Name Date 2,403,236 Spencer Sept. 24, 1946 2,071,515 FarnsworthFeb. 23, 1937

